Pfeiffer Vacuum

4.9.2 Application

Generating clean vacuum

Turbopumps are suitable for generating clean vacuum in the range of 10-3 to 10-10 hPa. Thanks to their high compression ratio, they reliably keep oil from the inlet area of oil-sealed pumps away from the recipient. Models with stainless steel housings and CF flanges can be baked out. This makes these pumps ideally suited for research and development applications where ultra-high vacuum requires to be attained.

Turbopumps can be used for evacuating large vessels with rotary vane pumps as backing pumps. In the case of turbo drag pumps, two-stage diaphragm pumps will suffice as backing pumps; however due to their low pumping speed, it will take them a great deal of time to pump down larger vessels. The gas throughput of this pump combination will also be highly restricted by the diaphragm pump. However this combination is an extremely cost-effective solution for a dry pumping station. It is often used for differentially pumped mass spectrometers and other analytical or research and development applications. If higher pumping speeds are required in the backing pump area, we recommend using multi-stage Roots pumps from the ACP series or, for chemical vacuum processes in the semiconductor or solar industry, the process-capable backing pumps.

Pumping stations consisting of a backing pump and a turbopump do not require valves. Both pumps are switched on at the same time. As soon as the backing pump has reached the necessary fore-vacuum, the turbopump quickly accelerates to its nominal speed and quickly evacuates the vessel to a pressure of $p$ < 10-4 hPa with its high pumping speed. Brief power failures can be bridged by the high rotational speed of the rotor. In the case of longer power failures, both the pump and the recipient can be vented automatically if the RPMs decline below a minimum speed.

The effects that play a role in evacuating vessels are described in Chapter 2. Dimensioning issues as well as the calculation of pump-down times are also described in that chapter.

Evacuating load lock chambers

Evacuating load-lock chambers definitely requires clean handling when transferring the workpieces to be treated in a vacuum process. If these items are channeled in from atmospheric pressure, the chamber should first be pre-evacuated via a bypass line. The running turbopump is then connected between the backing pump and the chamber via valves.

Analytical applications

In many cases, mass spectrometers are used in analyzers today. Fluids are often injected and evaporated in the inlet chamber of the vacuum system. Pressure is reduced in several stages, and the individual chambers are isolated from one another by orifices. Since each chamber must be pumped, the objective is to combine the gas flows via taps on the turbopump through the skillful combination of backing pumps and turbopumps. Specially modified turbopumps with taps are used for series applications. Besides the SplitFlow 50 described in Chapter 4.9.3, customer-specific solutions can be supplied.

Helium leak detectors, too, are equipped with turbopumps. In this case, the counter-flow principle is often used (see Chapter 7.2.1); i. e. a mass spectrometer is located on the high vacuum side of the pump. Due to the lower compression ratios of turbopumps for helium than for nitrogen or oxygen, the pump acts as a selective filter for helium.

Pumps with high gas loads in vacuum processes

The turbopump offers two advantages when pumping high gas loads for vacuum processes: It generates clean vacuum at the beginning of each process step, and can then pump down process gas without any harmful backflow. In the second step, the primary objective is to maintain a certain pressure at which the desired vacuum process should run. In this process, gas throughputs and working pressure will be determined by the application in question; i. e. a given volume flow rate will be pumped at a given gas throughput. Moreover, it should be possible to quickly achieve a clean intermediate vacuum when changing workpieces. Since these are conflicting requirements, a turbopump of sufficient size for the required gas throughput and the required intermediate vacuum must be selected. The process pressure will be regulated via an inlet valve (such as a butterfly valve). An example of how to dimension this kind of pumping station is shown in Chapter 2. The maximum permissible gas loads specified in the technical data should be taken to mean permissible continuous loads. This applies subject to the assurance of sufficient cooling in accordance with the specification and a backing pressure adjusted accordingly to below the maximum critical backing pressure.

Pumping corrosive and abrasive substances

When pumping corrosive gases, measures must be taken to protect the motor / bearing areas and the rotor, in particular, against corrosion. To do this, all surfaces that come into contact with corrosive gas are either provided with a coating or made from materials that can withstand attacks by these gases. A defined inert gas flow is admitted into the motor / bearing area in the fore-vacuum via a special sealing gas valve. From there, the gas flows through labyrinth seals to the fore-vacuum area, mixes with the corrosive gas and is pumped down by the backing pump together with the corrosive gas. In the case of pumps with bell-shaped rotors (e.g. the ATH M series), the sealing gas on the inner side of the Holweck stage can also act as convection cooling and increase the usable process window by reducing the temperature. Even in noncorrosive but dust-laden processes, sealing gas is an effective protection for the bearing and motor area.

The turbo-rotor blades can wear mechanically should dust accumulate; this could necessitate repairs and the replacement of the rotor. It should also be noted that deposits can be expected to form in the pump, which will shorten service intervals. In particular, it is necessary to ensure that deposits in the pump do not react to aggressive substances with moisture. Consequently, the pumps should be vented with dry inert gases only, and should be fitted with sealed fore-vacuum and high vacuum flanges when servicing is required. Turbopumps for these applications are either classic turbopumps without a Holweck stage, or turbopumps with a Holweck stage which would be a compromise between the critical backing pressure and the particle tolerance. Dust deposits in the Holweck stage resulting in blockage of the rotor can be reduced by increasing the gaps between the rotor and the stator in the Holweck stage. In an ATH M series turbopump, for instance, non-adherent dusts are primarily observed in the collecting channel near the fore-vacuum flange after long-term operation in a sputter application with particle content. The Holweck stage is still clean and the pump remains operational.

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